1. Field of the Invention
The present invention relates to an amplifier circuit and a light receiving element which are applicable to the field of optical communication.
2. Description of the Related Art
FIG. 10 shows a first conventional example of an amplifier circuit proposed as a first-stage amplifier circuit for a burst signal (see Patent Document 1).
In the first-stage amplifier circuit, an amplifier circuit 11, which converts an input current signal provided from a photodetector 10 into a voltage, a feedback resistor 101 connected between the input and output terminals of the amplifier circuit 11, and a switch 104 or 105 connected in series to a resistor 102 or 103 are connected in parallel.
A determining circuit 110 determines whether or not an output signal Vo amplified by the amplifier circuit 11 exceeds a reference voltage 111. If it is determined that the output signal Vo exceeds the reference voltage 111, the result of the determination is memorized into a memory element 112 or 113. The output of the memory element 112 or 113 is used as a control signal of the switch 104 or 105 and thereby a resistor 102 or 103 according to switching operation is connected to the feedback resistor 101 in parallel to vary the gain of the first-stage amplifier circuit.
In the above-described first-stage amplifier circuit, when the light receiving element 10 detects a pulsed light, a pulsed photo-current having an amplitude according to the intensity of the pulsed light is generated and inputted into the amplifier circuit 11. The inputted current is varied to the output signal Vo having a voltage amplitude according to the resistance value of the feedback resistor 101. The amplitude value of the output signal Vo is compared to the reference voltage 111 in the determining circuit 110. When the output amplitude value exceeds the reference voltage, the output of the determining circuit 110 is varied. Then, a VCC voltage is memorized into the memory element 112 and the output value of the memory element 112 is varied to close the switch 104.
When the switch 104 is closed, the feedback resistance value becomes the parallel resistance value between the feedback resistor 101 and the resistor 102, and thus the gain of the first-stage amplifier circuit is decreased. When the next pulse of the pulsed light is entered, a voltage amplitude value according to the parallel resistance between the feedback resistor 101 and the resistor 102 is outputted as the output signal Vo, which is compared to the reference voltage 111 in the determining circuit 110. If the voltage amplitude value of the output signal Vo exceeds the reference voltage 111, the VCC voltage is shifted to the memory element 113, the switch 105 is closed, and thus the gain of the first-stage amplifier circuit is decreased.
By controlling the switches 104 and 105 according to the intensity of the pulsed light, the gain of the first-stage amplifier circuit is varied so as not to saturate the output of the first-stage amplifier circuit. Thus, a wide dynamic range can be achieved and a rapid-response circuit structure can be obtained under simple control.
FIG. 11 shows a second conventional example of an amplifier circuit which does not use control of the opening and closing of switches (see Patent Document 2). The first-stage amplifier circuit includes an amplifier circuit 11 that an input current signal outputted from a light receiving element 10 is varied to a voltage, a feedback resistor 101 connected between the input and output terminals of the amplifier circuit 11, and a variable resistor 201 that a resistance value is varied according to a control voltage connected to the feedback resistor 101 in parallel.
When an input current signal is outputted from the light receiving element 10, a current detector circuit 202 generates a DC (direct current) voltage for generating a dummy direct current equal to the peak value of the input current and inputs the generated DC voltage to a dummy current generating circuit 203. The dummy current generating circuit 203 outputs a DC voltage Vp, which is obtained when the dummy direct current is inputted to the amplifier circuit 11. The current voltage Vp is inputted to an amplitude control circuit 204 and the current voltage Vp is compared to a reference voltage, thereby generating a control voltage Vc according to the difference of the compared value. The control voltage Vc becomes a control voltage of the variable resistor 201 and the parallel resistance comprised of the feedback resistor 101 and the variable resistor 201 is varied based on the control voltage. By controlling the variable resistor 201, the output amplitude of the amplifier circuit 11 becomes a constant value regardless of the amplitude of the input current signal.
As described above, the peak value of an input current is detected and the control voltage Vc of the variable resistor is generated. Accordingly, the value of the parallel feedback resistance can be controlled steplessly and an output voltage Vo of the amplifier circuit 11 that has a constant amplitude can be obtained. Further, as the first conventional example, the effect derived from switching noise can be ignored.
However, in a system that switches are connected to resistors in series and the opening and closing of the switches are controlled such as the first conventional example shown in FIG. 10, the accuracy of the feedback resistance value becomes lower unless the on-resistance of the switches is smaller than the resistance of the resistors connected to the switches in series. As a result, if the on-resistance of the switches is forced to be smaller by making a size of the switches larger, a problem may be occurred such as the effect of noise during switching or the reduction of switching speed.
FIG. 12 shows an example of noise generated during switching of switches in the first conventional example.
FIG. 12 shows the input current Iin outputted from the light receiving element 10, the output voltage Vo of the amplifier circuit, the opening and closing state (on-off state) of the switches 104 and 105. The assumption is that the value of the input current Iin is large and the gain is switched between two levels.
As can be seen from FIG. 12, the output voltage Vo is varied immediately after the switches 104, 105 are switched and thus switching noise is occurred. In this case, switching noise may be reduced to a certain degree by altering circuit structure. However, it is considerably difficult to completely eliminate switching noise as long as switches 104, 105 are opened or closed.
On the other hand, in the second conventional example shown in FIG. 11 that a control of the opening and closing of switches is not performed, the circuit structure is more complicated. In addition, response speed is limited because the peak value of an input current is detected and a control voltage is generated by feedback control due to the detected peak value.